At present it is conventional to coat optical fibers with two layers of coating (known as "dual-coating"). The first layer, which is in contact with the optical portion of the fiber, provides resilient elastomer cushioning. The second layer is hard and withstands abrasion, and it provides a barrier layer against humidity. At ambient temperature, conventional moduluses for the first and second layers are respectively 1 mega pascal (MPa) to 10 MPa and 200 MPa to 1000 MPa. The two layers are conventionally made of resin, in particular a urethane-acrylate resin. The two layers-are conventionally applied to a fiber in a fiber-drawing tower.
It is conventional for such a fiber-drawing tower to comprise in order:
a fiber preform unit; PA1 a first coating station at a temperature of about 20.degree. C. to 80.degree. C. for coating the fiber with a first curable formulation; PA1 a first curing station comprising a first series of polymerizing lamps, in particular ultraviolet (UV) lamps; PA1 a first cooling station for cooling the fiber coated in the first layer; PA1 a second coating station at a temperature of about 20.degree. C. to 80.degree. C. for coating the fiber with a second curable formulation; PA1 a second curing station comprising a second series of polymerizing lamps, in particular UV lamps; PA1 a second cooling station for cooling the fiber coated in the second layer; and PA1 a winding station (comprising a capstan and a winder). PA1 i) the fiber is allowed to cool naturally, by increasing the distance between the polymerization (UV) lamps and/or by increasing the distance between the last polymerization (UV) lamp for the first layer and the station for coating the second layer and/or the distance between the last polymerization (UV) lamp for the second layer and the final winding station; or PA1 ii) the fiber is subjected to forced cooling by using a cooling tube. PA1 a first coating step with a first coating composition and a polymerization step, said first composition comprising a curable formulation and a compound having a low vaporization temperature; and PA1 a second coating step with a second coating composition and a polymerization step, said second composition comprising a curable formulation. PA1 a first coating step with a first coating composition and a polymerization step, said first composition comprising a curable formulation; and PA1 a second coating step with a second coating composition and a polymerization step, said second composition comprising a curable formulation and a compound having a low vaporization temperature. PA1 a first coating step with a first coating composition and a polymerization step, said first composition comprising a curable formulation and a compound having a low vaporization temperature; and PA1 a second coating step with a second coating composition and a polymerization step, said second composition comprising a curable formulation and a compound having a low vaporization temperature.
Prior to the winding station, the tower may also comprise an "SZ" station serving to apply twist in alternating directions to the fiber in order to reduce the value of its polarization mode dispersion (PMD).
During the method of coating the two layers, polymerization generates a large amount of heat. This heat as given off gives rise to several problems. These problems comprise coating defects which generally give rise to large fluctuations in diameter and thus to an out-of-specification product that has to be scrapped. In addition, in some cases too high a temperature can slow down polymerization of the resin, thereby giving rise to a coated fiber whose properties vary significantly over time. Premature use of such a fiber, or aging thereof in a humid atmosphere, can result in a product that is defective. In addition, at the end of the first coating step that forms the primary layer, too high a temperature gives rise to problems when coating the second layer, since the resin of the second layer is not always properly applied if the temperature of the substrate is too high. In addition, if the second layer has a pigment, etc., the primary layer cannot continue polymerizing since the pigment forms a barrier preventing UV radiation from penetrating to the primary layer. It is then necessary either to polymerize the primary layer over a longer period of time which means either that the fiber-drawing speed must be reduced or else that a polymerization unit must be added, which in both cases increases the temperature of the fiber and thus the problems associated with temperature, or else it is necessary to increase the distance between the primary and the secondary coating steps, which, while not increasing temperature, does nevertheless require considerable space in the tower. At the end of the second coating step it is also necessary to limit the temperature so that the polymer of the second coating is not imprinted on passing through the capstan and/or the SZ station, since that could subsequently give rise to stresses.
In order to maintain the quality of the fiber coating and of the fiber itself, it is therefore necessary to control the temperature of the coating on leaving the steps in which the primary and secondary layers are polymerized. Such temperature control thus requires that a fraction of the tower is dedicated to cooling the fiber.
At present, two solutions are in use:
Solution i) is unsatisfactory since it takes up a large amount of space in the tower, which space becomes even greater with the higher fiber travel speeds presently in use.
Solution ii) is not satisfactory either, since it requires investment in additional equipment and it implies an increase in direct costs. Furthermore, to enable the cooling tube to be effective, it is necessary for it to have irises that come as close as possible to the fiber so as to remove the boundary layer which isolates the fiber from its surroundings and which puts a limit on heat exchange, and which therefore constitutes an obstacle to good cooling of the fiber. Unfortunately, when irises are too close to the fiber, there is a risk that they will touch the fiber, thereby increasing the amount of out-of-specification product that needs to be scrapped. Finally, the cooling tube needs to be cleaned regularly, thereby increasing the cycle time.